The present invention relates to the reconstruction of an information signal from a recovered digital signal in a digital communications system, and more particularly, to a novel device and method for improving the perceived signal quality using an adaptive frame erasure mechanism in which a frame may be erased even though an error detection mechanism indicates that no errors are present in the recovered digital signal.
Digital communication techniques have provided economic, robust ways of transferring or storing information signals. These techniques involve converting an information signal to a digital form, as a set of digital symbols, which are then transferred or stored in some representation or form that facilitates transmission over some medium or storage. Typically, the digital symbols are binary-valued, which are referred to as bits. The original information signal may be reconstructed by recovering the digital symbols or bits from the representation; the recovered digital symbols are then used by a processing unit to reconstruct the information signal.
This approach or process has been applied to a variety of audio and/or visual information signals. In speech communications systems, speech signals are converted to digital form using speech encoders, such as VSELP, LPC-RPE or the like. Recordings on compact discs, for example, store audio signals in a digital format. In video teleconferencing and video compression, video signals are compressed into digital forms using compression algorithms, such as those developed by JPEG and MPEG, for efficient transmission. Additionally, text may be converted into digital form for message service systems, such as e-mail and CD-ROM applications. All of these applications may be viewed as digital communications.
Referring to FIG. 1, in a typical digital communications system, the information signal, represented by Block 100, is divided into a series of short segments, referred to as frames 102. The portions of the information signal within each of these frames 102 are then converted into respective sets of digital symbols. These symbols are conveyed or transmitted for each frame using a certain representation, which depends upon the medium being used to transmit or store the digital symbols. For example, a frame 102 may represent twenty milliseconds of speech or a single image in a video signal. This twenty millisecond frame may be converted to a digital representation or form containing about two hundred sixty bits or digital symbols. These bits may be represented as or converted to radio waves by a communications transmitter, such as a digital cellular telephone or the like, and transmitted from an antenna of the transmitter.
When the digital symbols are communicated over some medium or written into or read from a storage device, errors may occur in recovering the digital symbol values. These errors may cause a degradation in the signal quality perceived by a listener or user of the communications system. To address this problem, error detection encoding may be applied to the digital symbols before the representation process. Typically, error detection encoding is applied to each frame 102 separately. When the signal is reconstructed, a decoding operation or error checking procedure is used to determine if there are any errors in the digital information symbols of a frame 102. If an error is detected, then the digital symbols in that frame 102 are not used to reconstruct the original information signal 100. This is commonly referred to as "frame erasure." When a frame erasure occurs, frame replacement procedures may be used to determine the disposition of the information represented by the digital symbols within the erased frame with regard to the reconstruction of the original information signal 100.
Accordingly, error detection decoding is used to determine if a frame 102 is erased. If the code in an error detection decoder detects that an error has occurred in the recovery of the digital representation of a signal, the frame 102 is erased; however, if the code within the error detection decoder does not detect that any errors occurred in the recovery of the signal, the frame 102 will be used to reconstruct the original information signal 100. When a frame 102 is erased, there may be a degradation in the reconstructed signal.
In conventional communications systems, the symbol or bit recovery process is designed to maximize the probability that the recovered digital symbols are the correct ones or substantially the same as the digital symbols before representation and transmission or storage. Accordingly, the number of symbol errors is minimized which minimizes the number of frame erasures. Techniques which improve the quality of the recovered error detection encoded symbols are often added to the symbol recovery process as a means of improving the quality of the reconstructed signal.
There are two problems associated with the signal recovery and reconstruction process. First, it is possible for the error detection decoding to indicate that no errors are present in the recovered digital symbols, when in fact, there are errors present. This is because error detection decoders are fundamentally limited in their ability to detect all possible error patterns. Additionally, in many applications, error detection encoding is only applied to a subset of the digital symbols within each frame. The error detection encoded symbols are usually those symbols that have the most impact on the reconstruction process because they are the most perceptible symbols or bits and are often referred to as such. While the error detecting code within the error detection decoder may correctly indicate that the subset of symbols are without error, there may be errors in the symbols not encoded by the error detection encoder. When symbols or bits that are in error are used to reconstruct the information signal, the quality of the reconstructed signal is perceived to be degraded by the user.
Secondly, the design approach of minimizing frame erasure rate assumes that erasing a frame causes a degradation in quality as perceived by a listener and/or viewer. If the frame erasure rate, however, is fairly low, the frame erasures are not perceived by the user, but significant bit errors on unerased frames may be perceived by the user.
It is, accordingly, a primary object of the present invention to provide a novel device and method for improving perceived signal quality in a communications system which is not subject to the foregoing disadvantages.
It is another object of the present invention to provide a novel device and method for improving perceived signal quality wherein a decision whether to erase a recovered frame of digital symbols in the reconstruction of an information signal is a function of the side information from the recovery process and previous frame erasure information, even though the error detection decoding indicates no errors are present in the recovered symbols or bits.
It is a further object of the present invention to provide a novel device and method for improving perceived signal quality wherein the decision to erase the frame is adaptively determined in response to previous frame erasure information.
These and other objects of the invention together with the features and advantages thereof, will become apparent from the following detailed specification when read with the accompanying drawings in which like reference numerals refer to like elements.